/*
 * Copyright (C) 2008 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.io;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Preconditions;

import java.io.IOException;
import java.io.Reader;
import java.util.Iterator;
import javax.annotation.CheckForNull;

/**
 * A {@link Reader} that concatenates multiple readers.
 *
 * @author Bin Zhu
 * @since 1.0
 */
@GwtIncompatible
@ElementTypesAreNonnullByDefault
class MultiReader extends Reader
{
    private final Iterator<? extends CharSource> it;
    @CheckForNull
    private Reader current;

    MultiReader(Iterator<? extends CharSource> readers) throws IOException
    {
        this.it = readers;
        advance();
    }

    /**
     * Closes the current reader and opens the next one, if any.
     */
    private void advance() throws IOException
    {
        close();
        if (it.hasNext())
        {
            current = it.next().openStream();
        }
    }

    @Override
    public int read(char[] cbuf, int off, int len) throws IOException
    {
        checkNotNull(cbuf);
        if (current == null)
        {
            return -1;
        }
        int result = current.read(cbuf, off, len);
        if (result == -1)
        {
            advance();
            return read(cbuf, off, len);
        }
        return result;
    }

    @Override
    public long skip(long n) throws IOException
    {
        Preconditions.checkArgument(n >= 0, "n is negative");
        if (n > 0)
        {
            while (current != null)
            {
                long result = current.skip(n);
                if (result > 0)
                {
                    return result;
                }
                advance();
            }
        }
        return 0;
    }

    @Override
    public boolean ready() throws IOException
    {
        return (current != null) && current.ready();
    }

    @Override
    public void close() throws IOException
    {
        if (current != null)
        {
            try
            {
                current.close();
            }
            finally
            {
                current = null;
            }
        }
    }
}
